Lasers transmitting in the infrared (IR) portion of the electromagnetic spectrum have become very important in recent years. Of particular importance are lasers transmitting in the mid-IR range, comprising wavelengths between 3 and 5 p.m. In this range, two atmospheric transmission bands exist, which are very useful for applications such as Light Detection and Ranging (LIDAR) systems, chemical sensing, free-space communications and IR countermeasures (IRCM). The first of these bands lies between about 3 and about 4.2 μm, while the second band lies between about 4.4 and 5.2 μm. The bands are separated by an atmospheric CO2 absorption peak which has high attenuation at about 4.3 μm.
Lasers which can transmit in these two bands are limited. The majority of such lasers are based on solid state or fiber pump lasers in the near-IR range between 1 and 2 μm and use an Optical Parametric Oscillator (OPO) crystal to convert the 1 to 2 μm pump into the longer 3 to 5 μm wavelengths by a nonlinear interaction in the crystal.
Periodically Poled Lithium Niobate (PPLN) is a highly nonlinear material which is very useful as an OPO for converting a near-IR laser pump to the mid-IR. Due to the very high nonlinearity of PPLN and the ability to achieve desired phase matching by periodically poling the material, laser/OPO systems based upon PPLN have very high wavelength conversion efficiencies. PPLN based systems also can be pumped by commercial continuous wave (CW) lasers rather than by the high-peak-power pulsed lasers typically used with OPO materials.
Unfortunately, laser/OPO systems based upon PPLN can only produce output having wavelengths up to about 4 μm due to absorption of higher-wavelength radiation in the PPLN crystal. Extending PPLN-based sources to the second atmospheric transmission band beyond 4.4 μm would be highly useful for the applications noted above such as LIDAR, chemical sensing, free-space communications, and IRCM, as well as many others.
The output from a PPLN can in turn be wavelength-converted to higher wavelengths, for example, by using an appropriate glass fiber to shift the wavelength the PPLN output before it is transmitted as the final output of the laser.
The magnitude of the wavelength shift in the radiation emitted by the glass fiber is based on the “phonon energy” of the fiber, having units of cm−1. The energy of the radiation, both pump and shifted wavelength, is inversely related to its wavelength, i.e.,
      E    =          hc      λ        ,where h is Planck's constant, c is the speed of light, and λ, is the wavelength of the radiation. Thus the change in wavelength λ, between the pump and emitted radiation corresponds to the phonon energy of the glass and is often called the “Raman shift” or “Stokes shift,” and the thus-shifted wavelength emitted by the fiber is often called the “Stokes wavelength.”
Chalcogenide fiber Raman lasers and amplifiers can operate in the mid-IR range. For example, U.S. Pat. No. 6,928,227 to Shaw et al., which shares inventors in common with the present invention and is hereby incorporated by reference into the presents disclosure, describes a Raman laser and amplifier based upon arsenic selenide (As—Se) glass fiber. Raman amplification and lasers using As—Se fiber also is described in P. Thielen et al., “Small core As—Se Fiber for Raman Amplification,” Optics Letters 28 [16] (2003) 1406-1408 and in S. D. Jackson et al., “Chalcogenide glass Raman Fiber Laser,” Appl. Phys. Lett., Vol. 88 (2006) 88. In the case of As—Se glass, the phonon energy of the glass is on the order of about 260 cm−1. The wavelength shift in this glass will correspond to this phonon energy. For example, for a pump having a wavelength of about 1.5 μm, the wavelength shift will be about 0.06 μm.